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Northrop grumman (Ts)

Radiation Hazard Analysis Procedure

Gabe Harms

9/12/2013

This document provides analysts with a step by step procedure in order to determine all radiation hazards that are present for any given device that emits electromagnetic radiation. All calculations have been deduced from other military documents including MIL-STD-464, NAVSEA OP 3565, IEEE C95.1-2005, and IEEE C95.6-2002

Radiation Hazard Analysis Procedure 09/12/2013

ContentsSCOPE..........................................................................................................................................................2

BACKGROUND.............................................................................................................................................2

PROCEDURE.................................................................................................................................................3

POWER DENSITY......................................................................................................................................4

FAR FIELD.............................................................................................................................................5

NEAR FIELD..........................................................................................................................................5

HERP ANALYSIS....................................................................................................................................5

HERF ANALYSIS....................................................................................................................................7

HERO....................................................................................................................................................8

APPENDIX A: NEAR FIELD POWER DENSITY CALCULATION........................................................................10

N CORRECTION FACTOR........................................................................................................................10

RECTANGULAR APERTURE.................................................................................................................10

CIRCULAR APERTURE.........................................................................................................................17

APPENDIX B: REFERENCES.........................................................................................................................18

Northrop Grumman 1Technical Sector


SCOPESince at the time of the tasking, no formal documentation on the topic was provided, research was done to find out what the guideline military documents should be used. MIL-STD-464, which is “Electromagnetic Environmental Effects Requirements for Systems”, was used as the primary source for this document. MIL-STD-464 references and recognizes the requirements of the following documents: NAVSEA OP 3565, IEEE C95.1-2005, and IEEE C95.6-2002. Therefore, all calculations derived in this document comes from one of the four documents mentioned above.

This document was created to provide personnel with enough information to be able to perform an analysis of a device emitting RF signals. First, how to find the device’s power density at a given distance will be explained. Then the safe and hazardous power densities will be provided in order to compare the device’s power to the restrictions. Completion of all steps in this document will provide the conclusion of whether or not any hazards of electromagnetic radiation to personnel (HERP), hazards of electromagnetic radiation to fuel (HERF), or hazards of electromagnetic radiation to ordnance (HERO) exist.

BACKGROUNDElectromagnetic radiation (EM radiation) is a form of energy emitted and absorbed by charged particles which exhibits wave-like behavior as it travels through space. EM waves have both electric and magnetic field components, which stand in a fixed ratio of intensity to each other, and which oscillate in phase perpendicular to each other and perpendicular to the direction of energy and wave propagation.

The way an EM field changes in character with distance from its source is described by Maxwell's equations. Electric fields produced by charge distributions have a different character than those produced by changing magnetic fields. Similarly, Maxwell's equations show a differing behavior for the magnetic fields produced by electric currents, versus magnetic fields produced by changing electric fields. For these reasons, in the region very close to currents and charge-separations, the EM field is dominated by electric and magnetic components produced directly by currents and charge-separations, and these effects together produce the EM "near field." However, at distances far from charge-separations and currents, the EM field becomes dominated by the electric and magnetic fields indirectly produced by the change in the other type of field, and thus the EM field is no longer affected by the charges and currents at the EM source. This more distant part of the EM field is the "radiative" field or "far-field," and it is the familiar type of electromagnetic radiation seen in "free space," far from any EM field sources (source).

Many electronic devices require the ability to communicate wirelessly in order to properly function. Radios, Radars, Phones, tablets and computers all have this characteristic. In order to be able to communicate wirelessly, these devices must be able to emit and receive signals from other devices.


http://en.wikipedia.org/wiki/Near_and_far_field


These signals are electromagnetic waves, and with modulation they are capable of carrying information long distances. However, every EM wave no matter the size must carry a certain amount of energy with it. The amount of energy that a wave has depends on its wavelength. Longer wavelengths like radio waves emit such low energy, that they do not cause hazards in most cases. While waves like UV waves carry so much energy that they cause burns to humans in very short time frames. The region of waves between these two types of waves can sometimes be tricky. In order to understand how much energy is carried, and some of the possible dangers, further investigation is required.

The most obvious dangers of EM radiation are the harmful effects they could have on humans. The energy in these waves could have negative impacts on a person’s health. We call this type of threat a Hazard of Electromagnetic Radiation to Personnel (HERP). But there are also other hazards that must be taken into account while considering EM radiation.

Extremely high power electromagnetic radiation can cause electric currents strong enough to create sparks (electrical arcs) when an induced voltage exceeds the breakdown voltage of the surrounding medium (e.g. air). These sparks can then ignite flammable materials or gases, possibly leading to an explosion (source). This is referred to as a Hazard of Electromagnetic Radiation to fuel (HERF).

Military ordnance, such as bombs, guns, and ammunition, are also at risk if they are electrically initiated devices (EID). EIDs are devices that use electrical current to produce an explosive, pyrotechnic, thermal, or mechanical output such as hot bridgewire (source). EM radiation could potentially set of these EIDs, and this is known as a Hazard of Electromagnetic Radiation to Ordnance.

The accidental firing of EIDs by RF energy is not a new concern. Commercial manufacturers of blasting caps have warned their customers for many years about the potential hazard involved in using electrically fired blasting caps in the vicinity of radio transmitters. Most EIDs employ a small resistive element called a bridgewire. When the EID is intentionally fired, a current pulse is passed through the bridgewire, causing heating and resultant initiation of the explosive charge. RF induced currents will cause bridgewire heating that may inadvertently fire the EID. Interface wiring to the EID generally provides the most efficient path for RF fields to couple energy to the bridgewire. However, RF energy can also fire extremely sensitive devices, such as electric primers, as a result of capacitive coupling from nearby radiated objects. RF energy may also upset energized EID firing circuits, causing erroneous firing commands to be sent to the EID. Non-bridgewire types of EIDs are being increasingly used for many ordnance applications (MIL-STD-464, pg. 82).

The rest of this document will provide instruction which will allow an analyst to determine whether or not these hazards exist.

PROCEDUREThe following set of instructions will provide a process that can be used by anyone to determine if any HERP, HERF, or HERO are present for the device in question. It will NOT provide any instruction to reduce these risks or hazards, but rather inform the analyst if they are present. The first step is to


https://acc.dau.mil/adl/en-US/436297/file/56641/MilHDBK240A.pdf

http://en.wikipedia.org/wiki/Electromagnetic_radiation_and_health#Fire_hazards


calculate the power density of the device at a specific distance, since we will use this power density for the HERP, HERF, and HERO analysis.

POWER DENSITYThis procedure is designed for an analyst to take a single device that emits EM radiation, and determine its power density at the desired distance from the device’s antenna.

Power density is calculated since most of the restrictions are limited based on power density. Since power density is dependent on distance, the minimum possible (worst case) distance should be used. This means that several power densities (e.g. distance to human, fuel tanks, explosives etc.) might have to be calculated in order to understand if all three hazards exist.

The Power Density equation is not the same for all distances. It varies depending on how close you are to the antenna due to the behavior of the near and far field regions of EM waves. In order to determine which power density calculation method we must use, the region (near or far) must be defined for the specific distance that the object/person is from the antenna. In order to do this, use the equation below (the following equations for power density have been discerned from the NAVSEA OP 3565):

The wavelength can be determined through the frequency (λ = speed of light/ frequency) while the dimensions of the antenna should be given by the spec sheet of the particular antenna. The result of this calculation will give you the boundary between the near and far field. Any distance greater than the result is considered the far field, and any distance less than the result is considered the near field. It can now be determined whether the distance of the object/human in question is in the near or far field.

Depending on whether the distance falls into the near or far field, one of two processes for power density will be used:



FAR FIELD

NEAR FIELD

The G and P unknowns should be provided by the antenna spec sheet, while r is specific to each calculation. For distances within the near field, the equation has an additional variable called the Near-Field Correction factor. This near field correction factor (N) requires much additional calculation in order to figure out. See APPENDIX A: NEAR FIELD POWER DENSITY CALCULATION for full calculation.

HERP ANALYSISNow that we have our power density, we can compare this value to the limits set by the IEEE C95.1-2005 and C95.6-2002 documents in order to tell us if there is in fact a HERP.

The IEEE defines two different scenarios: a controlled, and an uncontrolled environment. A controlled environment is when personnel are aware that the devices emitting radiation are on, and know that they are in close proximity to them. An uncontrolled environment is considered worst case scenario. This is when personnel are not aware that they are in close proximity to devices emitting radiation, or are unaware that these devices are on. It is important that radiation levels remain within the limits of the uncontrolled environment even though it might be a rare situation

3kH TO 300GHzThe following two tables are the limits set on devices that emit between the frequency range of 3kHz and 300GHz. Table 8 pulled directly from the IEEE C95.1-2005 document describes restrictions of the controlled environment, while table 9 are limits for an uncontrolled environment (more restrictive). This allows comparison between the power densities of your device against the restrictions set the 4th columns of table 8 and 9 (the power density we calculated is RMS).



If the power density exceeds the limits set in table 8, then there is definitely a HERP. If your power density exceeds the limits in table 9 then there is a less serious HERP, but a hazard still.



0kHz TO 3kHzThe following table limits devices that emit between 0kHz and 3kHz. Table 4 pulled directly from the IEEE C95.6-2002 document describes restrictions of a controlled environment and an uncontrolled environment. Before this table can be used, the power density that was calculated above must be converted into E-rms. To do this: sqrt(S*120π), where S is the power density. Now that you have E-rms, you can compare it to the restrictions set in the table below:

Once again, any E-rms value that exceeds the limits set in this table present a HERP.

HERF ANALYSISRF energy can induce currents into any metal object. The amount of current, and thus the strength of a spark across a gap between two conductors, depends on both the field intensity of the RF energy and how well the conductors act as a receiving antenna. Many parts of a system, a refueling vehicle, and static grounding conductors can act as receiving antennas. The induced current depends mainly on the conductor length in relation to the wavelength of the RF energy and the orientation in the radiated field. It is not feasible to predict nor control these factors. The hazard criteria must then be based on the assumption that an ideal receiving antenna could be inadvertently created with the required spark gap. Any area in the system where fuel vapors are present needs to have an analysis done on it (MIL-STD-464, pg. 81).

The NAVSEA OP 3565 states that the maximum safe radiation levels to fuel is 5 W/cm^2 from communication systems operating at 225MHz or more (NAVSEA OP 3565 section 6.3-3). Using the power density process described above, it is possible to compare to see if the power density is within the safe 5W/cm^2 in the area of the fuel (keep in mind that another power density will have to be calculated since the fuel is most likely not the same distance from the given antenna as a person). If it is not, then a HERF exists.

Antennas operating at a lower frequency than 225MHz and emitting less than 250 watts must be at least 50ft from fueling operations/fuel handling. Antennas operating at lower frequencies than 225MHz, but that are emitting more than 250 watts must be placed so that the power density



near the fuel is less than 0.009mW/cm^2 (NAVSEA OP 3565 section 6.3-3). If neither of these two conditions is met, then a HERF, again, exists.

For handheld communication transmitters, antennas radiating 10 watts or less shall remain at least 10 feet away from fueling/fuel-handling operations (NAVSEA OP 3565 section 6.3-3) (MIL-STD-464, pg. 81).

HERORF energy of sufficient magnitude to fire or dud EIDs can be coupled from the external EME (electromagnetic environment) via explosive subsystem wiring or capacitively coupled from nearby radiated objects. The possible consequences include both hazards to safety and performance degradation. Ordnance includes weapons, rockets, explosives, EIDs themselves, squibs, flares, igniters, explosive bolts, electric primed cartridges, destructive devices, and jet assisted take-off bottles (MIL-STD-464, pg. 82).

Adequate design protection for electro-explosive subsystems and EIDs must be verified to ensure safety and system performance. Unless a theoretical assessment positively indicates that the pick-up on EID firing lines or in electronic circuits associated with safety critical functions is low enough to assure an acceptable safety margin in the specified EME (bearing in mind the possible inaccuracies in the analysis technique), it will be necessary to conduct a practical test.

Verification methods must show that electro-explosive subsystems will not inadvertently operate and EIDs will not inadvertently initiate or be duded during handling, storage, or while installed in the system. For acceptance, it must be demonstrated that any pick-up in an EID circuit in the specified EME will not exceed a given level expressed as a margin in dB below the maximum no-fire threshold sensitivity for the EID concerned. More information about testing EID devices can be found in MIL-STD-464.

However, NAVSEA OP 3565 has determined the maximum acceptable exposure (MAE) to ordnance when there is no data available for the EID. This general MAE is will determine the closest distance ordnance can get without presenting a HERO. In order to use the equations provided by the NAVAIR OP, the power density must again be converted to electric field. To do this: sqrt(S*120π), where S is the power density.



Since we are unable to perform testing on any given EID, this table will allow an analyst to determine if there is a HERO, using equations which model a worst case scenario. If your calculation reveals that your EID is located inside of the maximum safe distance, then a HERO exists.



APPENDIX A: NEAR FIELD POWER DENSITY CALCULATIONThe following steps will allow you to calculate the power density of an object/human that is within the near field region of an antenna. The N correction factor must first be determined, and then a version of the power density equation for the far field region can be used.

N CORRECTION FACTORIn order to find the near field power density, we must find the N correction factor which is dependent on the distance to the antenna, and several other antenna characteristics. However, the calculation is different depending on the type of aperture associated with your antenna. For rectangular apertures see the section below. For circular apertures see the section after the rectangular aperture section.

RECTANGULAR APERTUREIn order to find the N multiplying constant, we must perform a graphical analysis. This will require us to have a couple specific numbers. First we must put the length and width of the antenna as well as the distance from the antenna in terms of our wavelength (λ). The first thing we must do is find what λ is:

Λ = speed of light/frequency

There is an intermediate step here that will tell us which graph we should be using. This involves finding the constant for estimating illumination (R) for both the vertical and horizontal dimension of the antenna. To find R the equation is:

Now R must be calculated in the vertical and horizontal dimensions using a constant BW of 3. Once you have Rv and Rw, continue the other calculations until these values are needed again. Now we must put the length and width of our antenna in terms of this λ:

Length = length/λ

Width = width/λ

Finally we must put the distance of the object/person from the antenna in terms of wavelength:



Distance = distance/λ

Now that we have all of the numbers that we need, we must use one of the graphs below to find our Gain reduction factor in both the length and width direction. Which of the graphs you use depends on your R value in each dimension of the antenna (vertical and horizontal which can be determined by the following table:

The estimated illumination (e.g. uniform or cos^3) will be different for each vertical and horizontal direction depending on your Rv and Rw values. The estimated illumination corresponds to a graph down below (pg. 12-16).

In order to use the graphs, use the distance of the object from the antenna in terms of λ on the x axis. Then use the length or width in terms of λ to move along the lines that cut through the graph diagonally to find two separate length and width Gain reduction factors.

Depending on your Rv and Rw values, table D-1 will tell you which graph to use to find each Gain reduction factor. For example if your Rv and Rw are 1.75 and 1.02 respectively, then you would use graph D-5 to find the vertical Gain reduction factor (Gv) and graph D-2 to find your horizontal Gain reduction factor (Gw).

This will result in two variables Gv and Gw in decibel units, derived from two graphs. This will allow us to find our N factor in terms of dB, but will then have to be converted to ratio form. N in terms of dB is as simple as the sum of the negative of our two reduction factors:

N(dB) = (-Gv) + (-Gw)

Now to get the final form of N, so that it can be used in the power density equation:

N(ratio) = antilog (N(dB)/10)



Now that you have your N correction factor, you can use the power density equation below for the near field region:



For clarification use the following provided example for a rectangular aperture









CIRCULAR APERTUREFirst we must calculate what is called the antenna illumination constant (R):

The beamwidth will always be 3, while diameter of the aperture (D) should be given by the spec sheet for a rectangular aperture. After finding R, you will use the following table to find out what the estimated illumination is.

Next you will calculate the Normalized on Axis Distance (X). The equation for X is as follows:

where d is the distance from the object to the antenna and D is the aperture diameter

Using this (1-r2)p equation and your value of X, you will finally find the Near Field Correction Factor (N) using the following graph:



Now that you have your N correction factor, you can use the power density equation below for the near field region:

APPENDIX B: REFERENCESThis section will contain references to all of the sources that were used in the creation of this document:



"C95.1-2005 - IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 KHz to 300 GHz." IEEE SA -. N.p., n.d. Web. 13 Sept. 2013.

"C95.6-2002 - IEEE Standard for Safety Levels With Respect to Human Exposure to Electromagnetic Fields, 0-3 KHz." IEEE SA -. N.p., n.d. Web. 13 Sept. 2013.

"Electromagnetic Radiation and Health." Wikipedia. Wikimedia Foundation, 09 Dec. 2013. Web. 13 Sept. 2013.

"Near and Far Field." Wikipedia. Wikimedia Foundation, 09 Jan. 2013. Web. 13 Sept. 2013.

United States. Department of Defense. MIL-STD-464, ELECTROMAGNETIC ENVIRONMENTAL EFFECTS REQUIREMENTS FOR SYSTEMS. N.p.: n.p., n.d. Print.

United States. Department of Defense. MIL-HDBK-240A, HAZARDS OF ELECTROMAGNETIC RADIATION TO ORDNANCE TEST GUIDE. N.p.: n.p., n.d. Print.

United States. Navy. Naval Sea Systems Command. NAVSEA OP 3565/NAVAIR 16-1-529, ELECTROMAGNETIC RADIATION HAZARDS (U) (HAZARDS TO ORDNANCE) (U). 16th ed. Vol. 2. N.p.: n.p., n.d. Print.

United States. Navy. Naval Sea Systems Command. NAVSEA OP 3565/NAVAIR 16-1-529, ELECTROMAGNETIC RADIATION HAZARDS (U)(HAZARDS TO PERSONNEL, FUEL AND OTHER FLAMMABLE MATERIAL) (U). 6th ed. Vol. 1. N.p.: n.p., n.d. Print.


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